There are many publications on methods and apparatuses for controlling/reducing occlusion.
Phonak in EP 2640095 proposes a method for tuning electro-acoustic parameters of hearing aids with an active control of the occlusion, that is, by emitting a counter sound in such a way as to cause a destructive interference. The occlusion effect control is built into the programming software of the hearing aids. The compensating filter of EP 2640095 is obtained starting from objective measurements on the subject carried out by means of a sensor (referred to as “channel microphone”) inserted into the apparatus and so that to sample the sound in the space remaining between the extremity of the apparatus and the tympanic membrane. These are measurements of REAG (Real Ear Aided Gain), i.e., the sound pressure in the ear canal having an apparatus introduced therein, the first measurement being performed when the apparatus is switched off and the second measurement being performed when the apparatus is switched on and used as a sound source. The acquired signal with the first measure is compared with a REUG signal (Real Ear Unaided Gain), i.e., a sound pressure in the ear canal of a free ear, which is not actually measured but instead “virtually” obtained by filtering the signal of the apparatus's microphone with a filter representing the gain of a free middle ear. That comparison results in the so-called Occlusion Effect Transfer Function (OE). The compensating filter (C) depends on the latter and on the Plant Transfer Function (P). P is the second REAG measurement (Real Ear Aided Gain) during which the apparatus is turned on and used as a sound source.
In the tuning phase of the electro-acoustic parameter of the apparatus, the compensation obtained (C) is applied in feedback mode and only downstream of the amplification required by deafness.
Notwithstanding that the neurosensory deficits related to a malfunction of the cochlea and/or auditory nerve (sensorineural o perceptual hearing loss) and to deficiencies in the brain, at the basis of a central deafness, cannot be solved completely by hearing aids, the application of hearing aids that do not alter the spectral cues of an audio signal give rise to the important result of allowing diseased cochlea to process a so-to-speak “physiological” audio signal, i.e., which corresponds to the one that naturally would be processed in the absence of a hearing aid, with consequent undoubted advantages for any types of hearing loss in terms of perceived audio signal quality and understanding.
By “physiological” we refer to the fact that the spectral cues of an audio signal that are used by the auditory system are also preserved, to determine the direction of origin and that contribute to speech intelligibility in a noisy condition.